1. Field of the Invention
The present invention relates to the improvement of waveform data processing in waveform measuring instruments using interpolated data, such as digital oscilloscopes.
2. Description of the Prior Art
The digital oscilloscope, which is a typical waveform measuring instrument, converts a time-series continuous signal waveform into digital data with an A/D converter, records them on a memory discretely, and displays the data recorded in the memory as waveforms.
If the sampling rate of the A/D converter is sufficiently high compared with the resolution of the display, signal waveforms can be reproduced by displaying only the points corresponding to these discrete data. However, the sampling rate is a finite value and so if the display resolution in the direction of the time base called T/div is expanded, the sampling rate is decreased relative to the resolution of the display and thus it becomes difficult to distinguish the signal waveforms from the reproduced display image.
In order to compensate for such relative decrease of sampling rate, a function for displaying waveforms by interpolating data between sampled data is provided. This function enables a waveform closer to the true signal waveform to be distinguished from the reproduced display image of an oscilloscope.
FIG. 1 is a block diagram showing an example of such conventional digital oscilloscopes.
Pre-amplifier 1 comprises the attenuation circuit and the pre-amplifier and adjusts the amplitude of input signals so that they come into an appropriate range for the input specifications of A/D converter (hereafter called “ADC”) 2 to output them to ADC 2.
ADC 2 converts input signals input from pre-amplifier 1 to digital data and outputs them to primary data processor 3.
Primary data processor 3 writes the digital data input from ADC 2 into primary memory 4 which functions as a buffer memory with a sampling rate meeting the time base setting. The data written in primary memory 4 are read by acquisition data processor 5 via primary data processor 3.
Acquisition data processor 5 writes the data read from primary memory 4 into acquisition memory 6 as well as applies averaging and addition, subtraction and multiplication or the like between two or more waveforms to the data read from acquisition memory 6. In addition, automatic measurement of waveform parameters, reading of values on waveforms by specifying with the cursor, or the like are also carried out in this acquisition data processor 5.
In display processing of the data written in acquisition memory 6, the data written in acquisition memory 6 are read by interpolation system 7 via acquisition data processor 5 and interpolation processing is carried out for the read data.
The data subjected to interpolation processing are input to display processor 8. Display processor 8 writes the display data into display memory 9 as well as outputs the display data in display memory 9 to display 10, such as a LCD, CRT, printer, or the like.
As described above, digital oscilloscopes not only display sampled data as waveforms but also have additional functions including a signal processing function such as averaging and an automatic waveform parameter measuring function. In conventional digital oscilloscopes as shown in FIG. 1, these functions are performed based on the sampled data.
FIG. 2 is a drawing illustrating automatic measurement of period in the configuration shown in FIG. 1. For automatic measurement of period, it is necessary to measure the time interval from point A where the waveform crosses the base line in one direction to point B where the waveform crosses the base line next time in the same direction as at point A. However, the data actually sampled and acquired in acquisition memory 6 are those for 14 points during two periods.
Although the number of data is less for the resolution of a display, sampling is done the number of times equal to or more than the Nyquist rate for the waveform frequency. Thus, a waveform is displayed on the screen equivalent to the original waveform such as that drawn with a continuous line if appropriate interpolation is carried out.
However, since values narrower than the time interval between actual adjacent sample points cannot be measured in measurements based on the acquisition data in the configuration of conventional digital oscilloscopes as shown in FIG. 1, an error occurs for the period Tm measured in the automatic parameter measurement against the original period Tr, as shown in FIG. 2.
FIG. 3 is a drawing illustrating noise rejection using averaging. If the number of sampling data points is less for the resolution of a display, the following problem occurs:
Consider a case where noise superimposed on signal waveforms shown with a broken line in FIG. 3 is to be rejected by averaging. It is assumed that points “∘” on the waveform shown with a broken line represent the data obtained by the first acquisition and points “X” represent the data obtained by the second acquisition. For two or more acquisitions, the sampling points in each time sampling have time-shifts relative to each other.
However, in the conventional configuration shown in an example of conventional digital oscilloscopes in FIG. 1, since sampled data at corresponding sampling points in each time acquisition are averaged as the data at the same instant respectively, the averaged waveform shows the waveform represented by symbols “Δ” and the continuous line in FIG. 3. This enables noise rejection to be performed. However, there is a problem that the frequency characteristic degrades in a high frequency region close to the Nyquist rate.